Summary Of Significant Accounting Policies (Carrying Value Of Assets Purchased) (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Electric plant in service		$ 8,025.8		$ 7,676.8
Accumulated depreciation		(3,117.2)		(2,982.2)
Production fuel		99.7		122.8
Materials and supplies		61.5		61.6
Carrying value of assets purchased		338.1	384.8
WPL [Member] | WPL's Edgewater Unit 5 Purchase [Member]
Electric plant in service	84.0
Accumulated depreciation	(50.0)
Construction work in progress	2
Production fuel	1.0
Materials and supplies	1.0
Carrying value of assets purchased	38.0
WPL [Member]
Electric plant in service		3,432.0		3,114.6
Accumulated depreciation		(1,326.6)		(1,243.8)
Production fuel		32.8		42.7
Materials and supplies		26.3		25.7
Carrying value of assets purchased	$ 38.0	$ 174.6	$ 201.8